R. E. Thoma

Phase equilibria in the system CsFZrF4

Abstract The phase equilibrium diagram of the condensed system CsFZrF 4 has been constructed from a combination of results obtained using thermal analysis, visual observation of crystallising fluoride mixtures, and quenching techniques. Phase identification was accomplished by petrographic and X-ray diffraction analysis. In the system CsFZrF 4 three intermediate compounds were isolated and identified: Cs 3 ZrF 7 , Cs 2 ZrF 6 , and CsZrF 5 . The compounds Cs 3 ZrF 7 and CsZrF 5 melt congruently at 784 and 518°C respectively; Cs 2 ZrF 6 melts incongruently to Cs 3 ZrF 7 and liquid at 530°C. Polymorphism is exhibited by Cs 2 ZrF 6 and CsZrF 5 , the latter compound showing a markedly exothermic inversion on cooling below 330°C. Crystals of the high-temperature equilibrium forms of Cs 2 ZrF 6 and CsZrF 5 were not retained on quenching; crystallographic verification of liquid-solid transitions was therefore unobtainable in the low-melting region of the system. The phase Cs 3 ZrF 7 was observed to vary in composition. As a homogeneous single phase, the solid solution extends to the composition 80CsF-20ZrF 4 (mole %). X-ray diffraction data obtained from single crystals of the stoichiometric phase revealed that the compound is face-centered cubic with a 0 = 9·70 A, and the space group is F 4 3m, F432, or Fm3m. This compound is apparently isotypic with K 3 ZrF 7 , (NH 4 ) 3 ZrF 7 , and K 3 UF 7 as regards the metal ions, but no deduction can be made from this work as to the exact arrangement of the fluoride ions.

Phase Diagrams of Binary and Ternary Fluoride Systems

Investigations of the equilibrium phase diagrams of metal fluoride systems have existed as a classical scientific effort since Gibbs, Roozeboom, et al. made their major contributions to science around the turn of the century. It remained for the workers in the Oak Ridge Aircraft Reactor Program and later in the Molten Salt Reactor Program to develop the means to acquire definitive data in massive amounts from a variety of interrelated techniques before the complex phase diagrams pertaining to the materials aspects of the Program could be defined with rapidity and precision. Because of their success in developing the experimental methods for investigation of the fluoride systems, the Oak Ridge group is responsible for a considerable fraction of the binary and ternary systems of the metal fluorides for which detailed phase diagrams appear in the literature.

Condensed equilibria in the uranium(III)-uranium(IV) fluoride system

Abstract The phase diagram of the condensed system UF 3 -UF 4 was established experimentally. The system is characterized by the occurrence of a single eutectic invariant point, at 32 mole% UF 3 , m.p. 865°C; by the absence of intermediate compounds; and by the behavior of UF 3 acting as crystalline solvent for U 4+ . Solid state equilibria were found to differ substantially from those reported previously by British and Russian workers. Uranium(IV) was found to undergo substitutional solid solution in UF 3 , reaching a maximum solubility at the solidus at UF 3 -UF 4 (67-33 mole%). As was to be expected analogous solution behavior with UF 4 acting as a solute for U 3+ , does not occur.

High-temperature phase equilibria in lithium-, sodium-, and thorium-fluoride mixtures☆

Abstract The equilibrium phase diagram for the lithium-, sodium-, thorium-fluoride ternary system was determined experimentally using standard laboratory methods. Crystallization reactions in LiF NaFThF 4 mixtures were found to result in the occurrence of fourteen singular and invariant equilibrium points at the liquidus, including three eutectics at the LiFNaFThF 4 compositions, 30−56·5–13·5−13·5, 43·5−32·5−5−24 and 54·5−13·5−32 mole%, and at the temperatures of 505, 509 and 525°C. Peritectic invariant points were found at temperatures as high as 825°C. The high-temperature modification of a crystalline phase, long assumed to be NaThF 5 , was identified as the hexagonal modification of the compound, 7NaF. 6ThF 4 , space group, P 3 c 1. In this phase lithium was found to exchange for sodium, to a saturation limit where lithium substitutes at approximately two-thirds of the sodium sites. Ternary compound formation was absent in the system. The results of the present study were the basis for a revision of the NaFThF 4 phase diagram.